如何将java对象转换成json数据

package cn.hopetesting.com.test;

import cn.hopetesting.com.domain.User;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;


/**
 * @author newcityman
 * @date 2019/9/17 - 23:12
 步骤1、导入jackson的三个包
 2、创建一个user对象，并赋值
 3、创建jackson的核心对象ObjectMapper
 4、调用ObjectMapper的writevalue等相关方法
 */
public class TestJson {
@Test
 public void test1() throws IOException {
//1、创建对象
 User user = new User("zmy",20,"male");
 //2、创建jackson的核心对象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 //3、转换
 /*try {
 String json = mapper.writeValueAsString(user);
 System.out.println(json);
 } catch (JsonProcessingException e) {
 e.printStackTrace();
 }*/
 //将数据写到d://a.txt文件中
// mapper.writeValue(new File("d://a.txt"),user);
 //将数据关联到writer中
 mapper.writeValue(new FileWriter("d://b.txt"),user);

 }

@Test
 public void test2() throws IOException {
//1、创建对象
 User user = new User("zmy","male");
 user.setBirthday(new Date());
 //2、创建jackson的核心对象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(user);
 System.out.println(json);

 }

@Test
 public void test3() throws IOException {
//1、创建user和list对象
 User user = new User("zmy","male",new Date());
 User user1 = new User("light",10,new Date());
 User user2 = new User("cityboy",5,new Date());

 List<User> ps = new ArrayList<User>();
 ps.add(user);
 ps.add(user1);
 ps.add(user2);
 //2、创建jackson的核心对象 ObjectMapper
 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(ps);
 System.out.println(json);

 }

@Test
 public void test4() throws IOException {
 Map<String,Object> map = new HashMap<String,Object>();
 map.put("name","zmy");
 map.put("age",15);
 map.put("gender","男");

 ObjectMapper mapper = new ObjectMapper();
 String json = mapper.writeValueAsString(map);
 System.out.println(json);
 }

//演示 json字符串转换为java对象
 @Test
 public void test5() throws IOException {
//初始化json字符串
 String json="{"gender":"男","name":"zmy","age":15}";
 //创建ObjectMapper对象
 ObjectMapper mapper = new ObjectMapper();
 //转换json字符串为java对象
 User user = mapper.readValue(json,User.class);
 System.out.println(user);
 }}